Add toggle for ijar in java_import

Problem: java_import has been unusably broken for years for JARs with Kotlin interfaces, since ijar strips out important information. This has caused multiple dependent projects (includng official Bazel ones) to abandon java_import in favor of rolling their own versions, which themselves contain issues that are getting fixed in java_import. Fragmentation is bad, and fragmentation of bugs and fixes is worse.
For more, see
https://github.com/bazelbuild/rules_jvm_external/blob/master/private/rules/jvm_import.bzl
bazelbuild#4549
bazelbuild#14966
bazel-contrib/rules_jvm_external#672

Temporary solution: Until such time as ijar is fixed for Kotlin, this adds a toggle that enables/disables ijar on java_import. This should unblock use of java_import for libraries that might contain Kotlin, so implementations can reunify.

It also restores java_import to a state where it works correctly by default. Per the user manual, ijars are a build performance optimization to allow caching of actions that use JARs whose implementations change frequenly. Imported (externally compiled) JARs shouldn't be changing very often, meaning that the build performance cost of disabling ijars for these prebuilt JARs should be relatively low. Therefore, the ijar toggle is off by default, so that the build is correct by default. But ijar is still available though the toggle, just in case someone is importing a Java-interface-only JAR that they change all the time.

src/main/java/com/google/devtools/build/lib/rules/java/JavaImport.java

@@ -32,6 +32,7 @@
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.collect.nestedset.Order;
 import com.google.devtools.build.lib.rules.java.JavaRuleOutputJarsProvider.JavaOutput;
+import com.google.devtools.build.lib.packages.Type;
 import java.util.LinkedHashSet;
 import java.util.Set;
 
@@ -202,7 +203,8 @@ private ImmutableList<Artifact> processWithIjarIfNeeded(
       RuleContext ruleContext,
       ImmutableMap.Builder<Artifact, Artifact> compilationToRuntimeJarMap) {
     ImmutableList.Builder<Artifact> interfaceJarsBuilder = ImmutableList.builder();
-    boolean useIjar = ruleContext.getFragment(JavaConfiguration.class).getUseIjars();
+    boolean useIjar = ruleContext.getFragment(JavaConfiguration.class).getUseIjars()
+        && ruleContext.attributes().get("use_ijar", Type.BOOLEAN);
     for (Artifact jar : jars) {
       Artifact interfaceJar =
           useIjar

src/main/java/com/google/devtools/build/lib/rules/java/JavaImportBaseRule.java

@@ -54,6 +54,13 @@ public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment envi
         for IDE plug-ins or <code>tools.jar</code> for anything running on
         a standard JDK.
         <!-- #END_BLAZE_RULE.ATTRIBUTE --> */
+        .add(attr("use_ijar", BOOLEAN).value(false))
+        /* <!-- #BLAZE_RULE($java_import_base).ATTRIBUTE(use_ijar) -->
+        If you import an oft-changing JAR that doesn't have a Kotlin interface,
+        you can enable this as a caching optimization.
+        Temporarily available until ijar is updated to properly handle Kotlin.
+        For more, see https://github.com/bazelbuild/bazel/issues/4549.
+        <!-- #END_BLAZE_RULE.ATTRIBUTE --> */
         .add(attr("neverlink", BOOLEAN).value(false))
         /* <!-- #BLAZE_RULE($java_import_base).ATTRIBUTE(constraints) -->
         Extra constraints imposed on this rule as a Java library.